A recent focus of our research has been the molecular characterization of the catalase peroxidase (katG) protein that has been shown to be associated with mycobacterium virulence as well as sensitivity to INH. Using site-directed mutagenesiis, we have established that specific katG mutations can cause a reduction in enzymatic activity resulting in resistance to INH. We have also established a system for evaluating the effort of specific katG mutations on the functional oligomerization of katG and shown the same katG mutations are transdominant in this system. Using gene deletion analysis, we have recently shown that the entire katG gene is needed for enzymatic activity and to exert a transdominant effect.